Electrical power systems represent one of the most important infrastructures of the modern world; as a result, security plays a significant role in their design and maintenance. Cascading blackouts occasionally remind us of the tremendous importance that security has in power systems. The role of security is to decrease the risk of possible future blackouts and ensure reliable operation at all times. A power system is a highly complex and unpredictable entity composed of equipment prone to failures. “Security” refers to the system's ability to meet predicted load even in the case of a component failure. The term “contingency” is used to refer to system operation in the event of an unplanned outage. The most common cause of outages is line tripping. Reliability criteria is a well-defined standard that specifies what a power system has to withstand in order to be considered secure. The reliability criteria most-commonly used in practice is the N−1 reliability criteria that requires the system to maintain substantially normal operating conditions in the event of outage of any one (hence “N−1”) component in the system. When the reliability criteria is met, the system is considered secure.
Obviously, the total number of contingencies in large-scale power system networks is very high. Contingency analysis (CA) tests each contingency individually for possible security violations. Preferably, an alternating-current (AC) power flow should be calculated for each contingency separately, but this process would take too long. This is why most CA algorithms use simplified formulations of AC power flow: decoupled AC and linearized real power-phase angle power flow (historically referred to as a direct-current (“DC”) power flow, though in a sense this could be considered a bit of a misnomer since the DC power flows define linearized relationships between real power flow injections and nodal phase angles in an electrical system such as an AC power grid; the term is used because the equations involved do not have sinusoidal terms after linearization is done). Additionally, the CA process is usually divided into two stages: contingency selection and contingency evaluation. Contingency selection tests each contingency in order to determine the most critical ones. Once they have been selected, it is possible to execute full AC power flow on a small subset of all contingencies, such as the most critical ones, to check for system violations. Contingency evaluation refers to identifying which preventive actions need to be performed in order to eliminate contingency violations.
Along with recent developments in technology resulting in increased computing power, there has been a corresponding increase of interest in integrating additional security features into existing algorithms such as Unit Commitment (UC) and Economic Dispatch (ED). This is how Security Constrained Unit Commitment (SCUC) and Security Constrained Economic Dispatch (SCED) have been developed. SCUC determines unit commitment sequence with dispatching for the next 24 hours while ensuring secure system operation for steady state conditions and for all contingencies. Testing all contingencies every hour significantly increases the overall complexity of the algorithm. Although available computational power is considered large, it still represents a limiting factor for SCUC/SCED algorithms executed on large-scale systems. Therefore, these algorithms require more intelligent contingency handling. Various approaches to improving the efficiency of CA have been proposed. A major issue in CA is that, despite the fact that a system experiencing an outage is very similar to the system prior to the outage, power flow has to be executed “from scratch” for every contingency, and power flow calculation for every contingency in a large system is extremely computationally demanding.